More information on the Data Center approved program can be found in the Atlassian licensing documentation.
On installation Atlas CRM requires 256 megabyte of additional ram. This is excluding any memory requirements due to usage scenario's (see below). By default Jira runs with 756 megabytes of ram (-Xmx in Java runtime terms). You should raise this by 256m before installing Atlas CRM.
Here is an example table with other common memory configurations:
Your current configuration
Before installing Atlas CRM
Each node requires enough free space in the local home to store the Atlas CRM index. The index grows with the number of entities in the system.
index size (m is megabytes, g is gigabytes, all by approach)
For more information on indexing, see below.
Additional storage is required to store files that are attached to CRM entities. These files are stored in the shared home. The shared home should be sized according to your storage needs, this greatly depends on your use of Atlas CRM.
Depending on your use of the system, you should size your memory and cpu accordingly. There is no general use of thumb for this. It greatly depends on the size of the CRM database and the usage patterns for viewing, searching and editing. Your system sizing should be done according to the system sizing instructions in the Atlassian documentation.
All nodes in the cluster build a local index. The indexes are kept in sync by sending cluster messages amongs nodes. When an additional node enters the cluster, it will have to catch up with indexing first before being fully functional in terms of searching and sorting.